1. Field of the Invention
The present invention relates, in general, to a method for machining a piston for a variable displacement swash plate type compressor, the pistons being fitted over the peripheral portion of a swash plate and reciprocated in the bores of a cylinder by the rotation of the swash plate owing to the rotation of a drive shaft. In particular, the present invention relates to a method for machining a pair of shoe pocket for a piston in a variable displacement swash plate type compressor, which is capable of cutting the shoe pocket, conforming to a single imaginary sphere, respectively on the opposed walls of a slot in the piston of the compressor.
2. Description of the Prior Art
A compressor constituting one of the main elements of an air-conditioning apparatus for automobiles is an apparatus that selectively receives power from an engine by the intermittence action of an electromagnetic clutch, converts refrigerant gas supplied from an evaporator to a state of high temperature and high pressure by means of a compressing process, and discharges the refrigerant gas to the condenser.
A swash plate type compressor is operated in such a way that a swash plate mounted around a drive shaft while being inclined and shaped in the form of a disk is rotated together with the drive shaft and a plurality of pistons fitted over the peripheral portion thereof are linearly reciprocated in the bores of a cylinder by the rotation of the swash plate. In the process of the reciprocating movement of the pistons, refrigerant gas is sucked into the compressor, compressed in the cylinder bore, and discharged to a condenser.
FIG. 1 is a sectional view showing a typical variable displacement swash plate type compressor that can variably compress refrigerant gas.
As illustrated in FIG. 1, the variable displacement swash plate type compressor comprises front and rear housings 1 and 2, a cylinder block 3 having a plurality of circumferentially arranged bores 3a and being positioned between the front and rear housings 1 and 2, a drive shaft 4 rotatably mounted across the interiors of the front housing 1 and the cylinder block 3, a circular lug plate 5 fixed around the drive shaft 4 and rotated together with the drive shaft 4, a swash plate 6 mounted around the drive shaft 4 and connected to one side of the lug plate 5 by means of hinge structures 5a and 6a, a plurality of pistons 8 fitted over the peripheral portion of the swash plate 6 via a pair of shoes 7 and respectively reciprocated in the bores 3a of the cylinder 3 by the rotation of the peripheral portion of the swash plate 6 owing to the rotation of the drive shaft 4, and a suction reed valve 10 and a discharge reed valve 11 selectively opening and closing the inlet 9a and outlet 9b of a valve plate 9 by pressure variation in the cylinder bore 3a. The variable displacement swash plate type compressor further comprises a control valve 12 for controlling the stroke of pistons 8 by regulating the pressure of the crank chamber 1a of the front housing 1, and the suction chamber 2a and discharge chamber 2b of the rear housing 2.
The variable displacement swash plate type compressor constructed as described above compresses refrigerant gas supplied from an evaporator and discharges it to a condenser.
When the drive shaft 4 is rotated by means of the power of an engine, the lug plate 5 fixedly fitted around the drive shaft 4 is rotated together with the swash plate 6 and the pistons 8 fitted over the peripheral portion of the swash plate 6 are reciprocated at a distance proportional to the inclination angle of the swash plate 6. In the process of the reciprocation of the pistons 8, refrigerant gas supplied from an evaporator is sucked from the suction chamber 2a through the inlet 9a into the bores 3a while each piston 8 is moved rearward (in a right direction in FIG. 1), and the refrigerant gas sucked into the bores 3a is compressed by the compressing action of the piston 8 and discharged to the discharge chamber 2b through the outlet 9b at a high pressure and, thereafter, discharged to a condenser(not shown) while the piston 58 reaches the bottom dead point and moves forward.
In the above refrigerant gas compressing process, the swash plate 6 performs a rotating movement, whereas the piston 8 fitted over the peripheral portion of the swash plate 6 and reciprocated by the rotation of the swash plate 6 performs a repeated linear movement back and forth.
Referring to FIGS. 1 and 2, the shoes 7 are semispherical bodies, each of the shoes 7 having a plane and a semispherical surface. The shoes 7 are respectively situated in shoe pocket 83 and 84 that are respectively formed on both opposed walls 81 and 82 of the slot 80 of the piston 8. Therefore, the shoes 7 are situated between the piston 8 and the swash plate 6 and are respectively slid on the shoe pocket 83 and 84 by the variation of the inclination angle of the swash plate 6 while the swash plate 6 is rotated, so that the variation of the inclination angle of the swash plate 6 is accommodated by the sliding of the shoes 7 and, consequently, the wobbling force of the peripheral portion of the swash plate 6 can be transmitted to the piston without hindrance.
Meanwhile, the shoe pocket 83 and 84 on which the shoes 7 are situated and which are formed on the opposed walls 81 and 82 of the slot 80 of the piston 8 should have curved surfaces corresponding to the semispherical surfaces of the shoes 7. In order to allow the shoes 7 to perform a sliding rotation, the curvature centers of the shoe pocket 83 and 84 should coincide with each other and the curvature centers of the shoe pockets 83 and 84 should coincide respectively with the curvature centers of the shoes 7. This means that the two shoe pockets 83 and 84 preferably conform to a single imaginary sphere I.
The present invention is concerned with a method for precisely machining two shoe pockets 83 and 84 on both opposed walls 81 and 82 of the slot 80 of the piston 8 so that the shoe pockets 83 and 84 conform to the imaginary sphere I.
The knotty point in precisely machining the shoe pockets 83 and 84 is that the two shoe pockets 83 and 84 should conform to the single imaginary sphere I having a spherical center at the center of the slot of the piston 8, but a cutter 100 having a spherical cutter blade cannot be inserted into the slot so as to machine the shoe pockets 83 and 84 because the space between the opposed walls 81 and 82 of the slot of the piston 8 is narrow in comparison with the diameter of the imaginary sphere I.
In order to overcome the problem, U.S. Pat. No. 6,053,081 issued on Apr. 25, 2000 discloses a method for machining a pair of shoe pockets in a compressor piston.
As illustrated in FIG. 2, the US patent discloses a method for machining a pair of shoe pockets 83 and 84, which conforms to the imaginary sphere I, on the opposed walls 81 and 82 of the slot 80 of a compressor piston 8, wherein the slot 80 is narrower than the diameter of the imaginary sphere I. In the method for machining a pair of shoe pockets 83 and 84, there is provided an asymmetrical cutter blade 101 that is, at its widest point, narrower than the slot as measured in the direction of the longitudinal axis L1 of the piston 8, wherein the cutter blade 101 generates a sphere if rotated about a cutter axis. One of the cutter blade 101 and the piston 8 is moved with respect to the other such that the cutter blade 101 enters the slot without contacting either of the opposed walls 81 and 82, wherein the cutter blade 101 has a center point that coincides with the center of the generated sphere, and wherein the generated sphere is the same size as the imaginary sphere I. The piston 8 is rotated about a machining axis L2 to simultaneously machine the shoe pockets 83 and 84, wherein the machining axis L2 intersects the center point of the cutter blade 101. One of the cutter blade 101 and the piston 8 is moved with respect to the other to remove the cutter blade 101 from the slot 80 without contacting either of the opposed walls 81 and 82.
However, in said US patent method, since the shoe pockets 83 and 84 conforming to the single imaginary sphere I can be machined only if the curvature center of the cutter 100 coincides with the rotating center of the piston 8 and the moving distance of the cutter 100 in the slot 80 of the piston 8 is to be accurate, there is difficulty in managing the machining dimensions. That is, since the piston is to be rotated about axis L1 while the cutter 100 is to be moved linearly along the axis L2, the shoe pockets 83 and 84 are apt to be biased to one side or may not conform to the imaginary sphere I if the relative position of the piston 8 and the cutter 100 with regard to each other or the moving distance of the cutter 100 is slightly deviated from a specified value. Further, in this US patent invention, it is not easy to get rid of unfavorable vibration of the piston during its rotation due to its asymmetrical feature. More further, in this US patent, it is not easy to move precisely the cutter such that its center is located on the rotation axis of the piston. As a result, there is difficulty in machining the piston so as to prevent the compressor from being operated improperly.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a method for machining a pair of shoe pockets for piston in a swash plate type compressor, which is capable of cutting shoe pockets, which conform to a single imaginary sphere, on the opposed walls of a piston slot by rotating a cutter having an arc-shaped cutter blade and, simultaneously, revolving it around the axis of the piston slot within the piston slot, thereby improving the dimensional precision of machining.
In order to accomplish the above object, the present invention provides a method for machining a pair of shoe pockets for piston in a swash plate type compressor so that the shoe pockets are respectively formed on the opposed walls of the slot in the piston of the compressor while conforming to a single imaginary sphere, comprising the steps of: moving a cutter provided with an arc-shaped cutter blade that has a rotation diameter less than the width of the slot; relatively revolving the cutter with regard to the piston by allowing the central axis of an actually revolved part to move along a circular orbit within the slot while rotating the cutter, so as to cut the shoe pockets to conform to the single imaginary sphere; and retreating the cutter out of the slot when the cutter completes one round revolution and is returned to its initial position.
In an embodiment, the relatively revolving of the cutter with regard to the piston may be performed by allowing the rotation axis of the cutter to move along the circular orbit within the slot while fixing the piston.
In another embodiment, the relatively revolving of the cutter with regard to the piston may be performed by allowing the piston to move along the circular orbit within the slot while fixing the rotation axis of the cutter.